The avermectin series of compounds are potent anthelmintic and antiparasitic agents against internal and external parasites. The natural product avermectins are disclosed in U.S. Pat. No. 4,310,519 to Albers-Schonberg et al., and the 22,23-dihydroavermectin compounds are disclosed in Chabala et al., U.S. Pat. No. 4,199,569. Administration of the avermectin compounds occur orally, parenterally or topically.
However, the conventional topical formulations do not provide acceptable efficacy against ectoparasites, especially against Chorioptes. Often times these formulations fail due to the lack of extended efficacy. Additionally, conventional formulations of current medicinal agents require a withdrawal period of a few weeks after application of the active compound before any milk can be withdrawn from dairy animals for human consumption.